This invention relates generally to heat transfer devices and air filtration devices, and in particular to heat exchangers, ventilators, and enthalpy exchangers along with air filters. The invention is particularly well-suited for air-to-air heat exchangers utilizing the regenerator principle.
Modern energy efficient construction employs air "tight" structures to restrict infiltration of outside air. Lack of infiltration or natural ventilation has resulted in inadequate indoor ventilation. Standard 62-1989 of American Society of Heating, Refrigeration, and Air conditioning Engineers Inc. (ASHRAE), Atlanta, Ga., states, "When infiltration and natural ventilation are insufficient to meet ventilation air requirements, mechanical ventilation shall be provided. The use of energy recovery ventilation systems should be considered for energy conservation purposes in meeting ventilation requirements." (Sec. 5.1.) Standard 62-1989 suggests 0.35 air changes per hour of continuous fresh air for living areas, but not less than 15 Cubic Feet per Minute (CFM) per person based on design occupancy. For a 2500 square-foot home, this equates to about 120 CFM.
Bringing outside air into a structure for ventilation purpose can be problematic and expensive. Utilizing gas or electric heat to preheat separate ventilation air in winter is inefficient. For example, if the outside air is 20.degree. colder than indoors, approximately 1.2 kW of heat is required to preheat the 120 CFM of required ventilation for a 2500 square-foot home. Use of a heat recovery ventilator is by far the most efficient way to ventilate, exchanging as much as 85% of the heat from warm (inside) exhaust air with the cool fresh air. In summer, use of a heat recovery ventilator also reduces air-conditioning load by exchanging cool dry exhaust air with warm humid fresh air. An "enthalpy" exchanger has been found to be particularly effective in humid climates.
Another problem with bringing in ventilation air concerns the quality of the air. In many places, allergens, such as, pollen or mold spores, and/or other particulates such as soot from vehicle exhaust or emissions from industrial sites, exist in the outside fresh air through much of the year. Filtering these allergens and/or particulates out of the ventilation air is important for severe allergy or asthma sufferers. Air cleaning devices are known which will remove particulates of a certain size. Of particular interest is the High Efficiency Particulate Air (HEPA) filter available from Columbus Industries, Ashville, Ohio. A HEPA filter must capture 99.97% of all particles down to 0.3 micron in diameter.
Some prior art air-to-air heat exchanger technology for home use utilize a cross-flow heat exchanger core, e.g., Lifebreath.TM. heat recovery ventilator by Nutech Energy Systems, Inc. of London, Ontario, Canada; TherMax TW Model room ventilators made by Thermax Energy Recycling Ventilation Systems, Division of Kooltronic, Inc. of Hopewell, N.J.; NewAire.TM. air-to-air heat exchange ventilators made by Altech Energy of Madison, Wis.; U.S. Pat. No. 4,512,392 (Van Ee et al.) and U.S. Pat. No. 5,273,105 (Drake). A disadvantage of these devices is low heat exchanger effectiveness. The best theoretical effectiveness is approximately 70% for a cross-flow core. Practically, these devices only achieve a fraction of that effectiveness.
Other prior art technology includes the use of a rotary heat recovery, wheel, e.g., Honeywell "Perfect Window" System energy recovery ventilator, available from Honeywell, Inc. of Golden Valley, Minn. This device employs a rotating regenerative wheel, as well as a fresh air filter and a room air filter. Two types of rotary heat recovery wheels may be used--a desiccant wheel to transfer moisture and also dry heat, or a sensible wheel to transfer only dry heat. (However, as is known in the art, the sensible wheel will transfer moisture when the air drops below the dew point temperature as the air passes through the regenerative wheel.) An advantage of this technology is that high heat exchanger effectiveness is possible. A disadvantage is that it requires an additional moving part, i.e., the regenerative wheel. This regenerative wheel (rotary heat recovery wheel) is approximately 16 inches in diameter for one model. It rotates at about 30 RPM. On one side of the wheel there is outside air. On the other side, there is indoor air. A brush seal is used around the rim of the wheel, and in freezing conditions, warm moist air flowing past the seal will condense and freeze forming frost. If the frost melts, it may migrate to the rim of the wheel and refreeze which can cause the wheel to freeze up. To prevent wheel freeze up, an electric preheater on the incoming air is used to warm the air to 5.degree. F.(-15.degree. C.).
Yet other prior art technology which uses fixed, rotating or reciprocating heat exchanging beds or some method of periodically changing the airflow direction includes U.S. Pat. No. 3,978,912 (Penney et al.); U.S. Pat. No. 4,049,404 (Johnson); U.S. Pat. No. 4,391,321 (Thunberg); U.S. Pat. No. 4,493,366 (Ekman); U.S. Pat. No. 4,589,476 (Berner); U.S. Pat. No. 4,665,805 (Ekman); U.S. Pat. No. 4,688,626 (Tengesdal); U.S. Pat. No. 4,744,409 (Berner); U.S. Pat. No. 4,754,806 (Astle); U.S. Pat. No. 4,815,522 (Thunberg); U.S. Pat. No. 4,952,283 (Besik); U.S. Pat. No. 5,002,116 (Hoagland et al.); U.S. Pat. No. 5,050,667 (Berner et al.); U.S. Pat. No. 5,375,649 (Nilsen et al.) and D. A. Reay, "Heat Recovery Systems"(E.& F. N. Spoon, London, UK, 1979, pp. 17-35).
Most of the present air ventilation/heat recovery technology are large, heavy, bulky devices which are expensive, difficult to install, and complex, sometimes requiring preheating incoming cold air. Not withstanding the many known practical design problems for air-to-air heat exchangers, the art has not responded to date with the production of a compact, lighter weight, air-to-air heat recovery ventilator using a regenerative heat exchanger and not requiring any heater to heat incoming air to avoid freeze-up problems in the heat recovery ventilator, and also not requiring complex rotating seals in the regenerative heat exchanger between an indoor climate and an outdoor climate. The system of the present invention preferably utilizes a HEPA filter to provide high quality ventilation air.